When plants are grown in a greenhouse, an increase in the photoperiod, as well as continuous lighting, is one of the ways to improve plant productivity and energy savings. However, a number of crops under long photoperiods develop signs of light damage to leaves, and productivity is reduced. We studied the effect of the photoperiod (8, 12, 16, 20, and 24 h) and photon flux densities (60, 120, and 160 micromol/m2 with PAR) on cucumber plants Cucumis sativus L. in a prereproductive period. We show that the response of the cucumber plants to a photoperiod duration of more than 20 h, including continuous lighting, depending on the plant age and lighting conditions, may include epinastic reaction of the leaves, activation of a mechanism of nonphotochemical chlorophyll fluorescence quenching, and/or reversible photoinhibition of a reaction center of photosystem II, development of reversible chlorosis, reduction of a light-harvesting complex, and increase in the content of carotenoids. Reaction of immature and virginile plants to long photoperiods was different, which highlights the need for experimental separation of the prereproductive period of development in terms of age states and consideration of this when preparing programs of cultivation.